1. Field of the Invention
The present invention relates to a shaft seal, arranged between a shaft and a housing, having at least one sealing lip which bears with a sealing-lip supporting surface against the shaft surface moving relative to the seal. A predetermined surface pressure is applied to the sealing lip and shaft surface interface by a pre-stressing element.
2. Background and Summary of the Invention
In the prior art, a multitude of sealing arrangements for providing a seal between two components moving relative to each other has been disclosed. The sealing arrangement seals spaces containing different media or pressure conditions-from each other.
FIG. 1 shows diagrammatically, by way of example, such a sealing arrangement according to the prior art used in a field of use, which is frequently encountered. In the case illustrated, a gap between a shaft 1 and a housing part 2 is shown, through which shaft 1, rotating relative to the housing 2, is guided. These are to be sealed in such a manner that a medium contained in a space 3 cannot pass into a space 4, and vice versa. Arranged in space 3 is a shaft bearing 5, for example a grooved ball bearing, which supports shaft 1 in the opening of the housing 2 and is lubricated by a suitable lubricant. In contrast, space 4 is exposed to environmental influences and so spray water and dirt can pass into it, the intention being to prevent them from entering into space 3.
To mutually seal spaces 3 and 4, there is provided in the gap, which is defined by a shaft surface 7 and a surface 8 of the housing 2, a shaft seal. Shaft seal 10 consists of an elastomeric sealing material and has essentially a U-shaped cross-sectional configuration. A first limb 11 of the shaft seal 10 bears in a sealing manner against surface 8 of housing 2 and is secured thereonxe2x80x94for example, by means of compression. A second limb 12 is in sealing contact with shaft surface 7. First and second limbs 11 and 12, respectively, are connected to each other by a base section 13. Furthermore, an L-shaped stiffening element is provided for reinforcing first limb 11 and base section 13. In addition, shaft seal 10, according to FIG. 1, has a protective lip 15 which is arranged at that end of second limb 12 which is adjacent to base section 13, and comes to bear against shaft surface 7.
Second limb 12 bears with its outer end against shaft surface 7 at a predetermined contact pressure force, with the result that shaft surface 7 can rotate relative to shaft seal 10 and can also move in the axial direction. The contact pressure force is determined by the restoring force, which depends on the elastic properties of the seal material and the pre-stressing of second limb 12 with respect to shaft surface 7, and/or by the tangential force of a helical tension spring 16 which is inserted into shaft seal 10 at the outer end of second limb 12 and presses the outer end of second limb 12 against shaft surface 7 with a predetermined force.
A variant of shaft seal 10 according to FIG. 1, in which a protective lip 15 is not provided, is illustrated in cross section on an enlarged scale in FIG. 2. In particular, a sealing lip 20, formed at the outer end of second limb 12, can be seen in FIG. 2.
A body element 21 of sealing lip 20 is defined by two side surfaces 22 and 23 tapering toward each other. Body element 21 has an essentially triangular cross section, against which spring 16 presses, as is indicated by an arrow in FIG. 2. At the tip of body element 21, sealing lip 20 bears with an annular, relatively narrow sealing-lip supporting surface 24 against shaft surface 7.
Grinding seals of this type, such as shaft seal 10 described above, provide a reliable seal, particularly if the wear of sealing lip 20 is small on account of the surface quality of the shaft surface 7 and/or on account of the lubrication of the sealing edge or sealing-lip supporting surface 24. In contrast, increased friction of the seal on opposite surface 7 has a disadvantageous effect on account of the temperature increase associated therewith and the effects of wear caused as a result. The wear leads to a reduced sealing action of shaft seal 10, which, as will be explained in greater detail with reference to FIGS. 3A to 3C, 4 and 5, depends substantially on the surface pressure at sealing-lip supporting surface 24.
FIGS. 3A, 3B and 3C each illustrate profiles of sealing lips 27, 28 and 29, which are arranged on a sealing body 25, bearing against an opposite surface 26. They differ in xe2x80x9csharpnessxe2x80x9d. These profiles have been used for the measurements illustrated in FIGS. 4 and 5. The profiles of FIGS. 3A, 3B and 3C differ in each case merely by the different point radii R1, R2 and R3 of sealing lips 27, 28 and 29, respectively. Sealing lip 27, according to FIG. 3A, has a point radius R1=0.1 mm; sealing lip 28, according to FIG. 3B, has a point radius of R2=0.2 mm; and, sealing lip 29, according to FIG. 3C, has a point radius of R3=0.3 mm. The remaining parameters of the profiles: the rectangular cross-sectional shape of sealing body 25 with a height HO and a length LO, the overall length L1, the 30xc2x0 angle with respect to the axis of symmetry of the side surfaces 22 and 23 of the sealing lips, and the sealing materials are essentially identical for all of the profiles of FIGS. 3A to 3C.
FIG. 4 shows the distributions of the surface pressures in the sealing gap XR1, XR2 and XR3 for the different point radii R1, R2 and R3, respectively. FIG. 5 illustrates the rise in the maximum value of the surface pressure in the sealing gap YR1, YR2 and YR3 for the point radii R1, R2 and R3, respectively, as a function of compression distance.
The curves according to FIGS. 4 and 5 show a marked dependence of the surface pressure in the sealing gap on the point radius, the maximum value of the surface pressure decreasing with increasing point radius R1xe2x86x92R2xe2x86x92R3. In the case of relatively small point radii, a relatively large surface pressure is obtained. Accordingly, sealing action improves with the geometry otherwise unchanged. Laboratory tests also show that the sealing lips having the smallest point radii have the highest seal tightness. It should also be noted that the variants having the smallest point radii achieve the greatest surface pressures with, at the same time, the smallest reaction forces.
In summary, it follows from this that seal tightness of a sealing arrangement having a resilient sealing material depends substantially on the surface pressure in the sealing gap, which in turn depends on the xe2x80x9csharpnessxe2x80x9d of the sealing lip and the contact pressure force.
The wear of sealing lip 20, which influences the surface pressure of sealing-lip supporting surface 24, is therefore critical to the service life of the sealing arrangement or shaft seal 10. The wear depends on the relative speed of sealing lip 20 with respect its respective shaft surface 7, on the roughness of shaft surface 7 bearing against sealing lip 20, and on the wear properties of the sealing material.
The roughness of shaft surface 7 is reduced over time by sealing lip 20 rubbing against it, since the sealing lip grinds in a running surface on shaft 1. Even after a short running time, sealing lip 20 produces a finely polished region on shaft surface 7. Subsequently, sealing lip 20 is subject to a greatly reduced wear or virtually no at all. In the case of structures with small axial relative displacements of shaft surface 7, this region is very narrow. A structure of this type can be achieved if, for example, shaft seal 10 on shaft 1 is situated directly next to a fixed shaft bearing, such as, for example, a tapered roller bearing.
In the case of structures in which shaft seal 10 on shaft 1 is far away from a fixed bearing of a fixed/moveable bearing arrangement, a relative displacement between shaft 1 and shaft seal 10 in the axial direction of shaft 1 may occur. On account of different coefficients of thermal expansion of the shaft material and the housing material, axial relative displacements occur in the case of changing external temperatures during warm-up operation. Furthermore, axial relative displacements between shaft 1 and shaft seal 10 are caused by axial forces, which can arise due to intermeshing. These axial forces cause deformation of the housing and of the bearing cover and the bearing plate of a shaft bearing and thus also deformation between shaft seal 10 and a fixed bearing. This has a disadvantageous effect on the sealing arrangement since the deformation leads to axial displacement of the running surface of sealing lip 20 on shaft 1.
Such axial displacement of shaft 1 causes the running surface of sealing-lip supporting surface 24 on shaft surface 7 to change, with the result that sealing lip 20 is constantly in frictional contact with various circumferential points of shaft surface 7. This has the result that, in comparison to a structure without significant axial displacement of shaft 1, a substantially larger region of shaft surface 7 has to be ground in.
An axial relative movement between sealing lip 20 and shaft 1 causes increased wear as a function of the magnitude and frequency of the axial displacement. As a consequence of such wear, a completely rounded, worn contour of an originally pointed sealing lip can arise, as illustrated in FIG. 6.
FIG. 6 shows, from left to right, various states I, II, III and IV of sealing lip 20.
State I refers to sealing lip 20 when new, with an originally pointed sealing-lip supporting surface 24. The narrow region on shaft 1 is polished by sealing-lip supporting surface 24 and is indicated by a line in FIG. 6. State II shows sealing lip 20 with a sealing-lip supporting surface 24xe2x80x2, said lip having already been worn to a certain extent without there being axial play of shaft The region polished by the sealing lip 20 corresponds essentially to the width of sealing-lip supporting surface 24xe2x80x2.
When there is axial play of the shaft 1, after a short running time, state III arises in which a sealing supporting surface 24xe2x80x3 has formed. The region polished by sealing 20 is indicated on the shaft 1 and is wider than sealing-lip supporting surface 24xe2x80x3.
This region is wider because of the axial play of shaft 1. This is associated with a relatively pronounced wear or relatively pronounced widening of the originally pointed sealing-lip supporting surface 24 (state 1). After a long running time state IV finally arises, in which the wear of sealing lip 20 has produced a greatly widened sealing-lip supporting surface 24xe2x80x2xe2x80x3. The region polished by sealing lip 20 is wider than sealing-lip supporting surface 24xe2x80x2xe2x80x3.
The seal tightness of the sealing arrangement is disadvantageously affected in going from Ixe2x86x92IIxe2x86x92IIIxe2x86x92IV in two respects: firstly, there is enlargement of sealing supporting surface 24xe2x86x9224xe2x80x3xe2x86x9224xe2x80x2xe2x80x3. Secondly, the material thickness of sealing lip 20 is reduced perpendicular with respect to the sealing-lip supporting surface (indicated in FIG. 6 by means of a dashed line parallel to shaft surface 7), as a result of which the contact pressure force produced by the elasticity of the sealing-lip material and by the pre-stressing of spring 16 is reduced. The consequence of this is a pronounced loss of surface pressure and therefore of seal tightness.
The wear also depends on supplying the contact point with lubricant. For this purpose, the opposite running surface or shaft surface 7 is generally greased in advance. However, during installation of shaft seal 10 or of shaft 1 the grease is wiped away, with only a small amount of grease remaining, for example, in the chamber between protective lip 15 and sealing lip 20 in FIG. 1. Furthermore, the lubricating grease is increasingly pushed to the outside from the region of the contact surface or running surface of shaft seal 10 by a constant axial movement.
In the prior art, it is known to polish opposite surface 7, against which sealing lip 20 bears, to reduce the surface roughness to reduce wear of sealing lip 20. The sealing lip then no longer has to ensure a corresponding grinding-in process. However, a surface treatment of this type is subject to process fluctuations and requires additional machining time and increased costs.
Another known approach is avoidance of combined rotational and linear movement in the sealing region. For example, DE 198 39 485 A1 discloses a sealing arrangement in which a sealing lip bears against a sleeve rotating together with the shaft. In the event of axial movement, the seal is carried along via a bearing to the shaft, with the result that the running surface of the sealing lips on the sleeve does not change. However, a structure of this type is very complex and requires the shaft bearing to be connected to the seal. Thus, the actual seal cannot be used independently of the shaft bearing.
DE 198 31 523 A1 discloses a further possibility of avoiding at least small axial movements of an opposite surface relative to a sealing lip. An axial projection, which has the sealing lip, on a sealing body, is pre-stressed in the axial direction by means of a compression spring which is arranged between the sealing body and a stop situated opposite the latter. The axial compression spring is intended to absorb high-frequency, small-amplitude oscillations of the opposite surface in the axial direction without relative movement between the sealing lip and the opposite surface. However, the costs of the seal are significantly increased by the additional, axial compression spring.
It is also known to treat the surface of the sealing lip to reduce friction as disclosed, for example, in DE 199 49 205 A1, in which treatment of a moveably arranged sealing-lip surface with halogens is described.
Furthermore, it is known in the prior art to coat the sealing lip with a friction-reducing material. DE 198 39 502 A2 shows, for example, a sealing lip coated with PTFE.
However, reducing the friction of the sealing lip, according to the prior art, requires coating of or a hardening treatment of the sealing-lip material. Both processes increase production costs of the seal. Moreover, the coating may become detached from the base material of the sealing lip. Furthermore, a hardening treatment is subject to process-induced fluctuations in quality.
Drawbacks of prior approaches are overcome by a shaft seal with at least one sealing lip which bears with a sealing-lip supporting surface against an opposite surface moving relative to the seal at a predetermined surface pressure applied by a pre-stressing element. The sealing lip has a main body element and at least one projection, which is supported by and adjacent to the main body element. The projection bears against the opposite surface and is a sacrificial element, which abrades due to friction with the opposite surface. In the process, the projection polishes the opposite surface. The radial cross-sectional profile of the projection is such that the sealing-lip supporting surface is not substantially enlarged by abrasion thereby maintaining a predetermined surface pressure at the sealing-lip surface of the projection over a service life of the seal.
An advantage of the present invention is that because the sealing-lip supporting surface does substantially enlarge, the surface pressure does not diminish. Thus, over the life of the seal, seal tightness is maintained.
Yet another advantage of a seal, according to the present invention, is that relatively small demands can be placed on the opposite surface moving relative to the seal or on the running surface of the seal. Because of the shape of the projection, some axial movement can be tolerated.
Another advantage of the present invention is that it is inexpensive to produce. The opposite surface need not be polished to such a high degree as prior systems. The application of a lubricating material is also not critical to the seal.
The invention makes provision for the projection to be a sacrificial element, which abrades due to friction with the opposite surface. In the process, the projection polishes the opposite surface. In this case, the radial cross-sectional profile of the projection is selected in such a manner that the sealing-lip supporting surface is not substantially enlarged with increasing abrasion of the projection. Thus, a predetermined surface pressure at the sealing-lip supporting surface of the projection is essentially maintained over the service life of the seal. This means that the projection is of rather more pointed design in comparison with the main body element. In contrast, the main body element is of such a designxe2x80x94is preferably wider that the projection is supported and thus protected against lateral movement.
An essential concept of the present invention therefore resides in recognizing that abrasion of the seal occurs and ensuring that the cross-sectional geometry due to abrasion causes a smaller enlargement of the sealing-lip supporting surface than is the case in prior art seals. As a result, the surface pressure remains essentially the samexe2x80x94apart from a slight decrease due to the abrasion-induced change in the seal radius, with the resultxe2x80x94as explained abovexe2x80x94that reliable sealing is ensured with long-term stability.
Preferably, the radial extent of the projection is selected so that abrasion of the projection over the service life of the seal essentially stops due decrease in friction on account of the increasingly more finely polished opposite surface. The precise dimension of the radial extent of the projection also depends, inter alia, on how much axial displacement between the seal and opposite surface are to be reckoned with and how long it takes until a sufficiently fine polish is achieved over the entire region of axial displacement. The longer the projection, the wider it should be to provide sufficient stability with respect to lateral movement. The radial extent of the main body element is preferably greater than the radial extent of the projection. This ensures that the main body element, with its relatively large volume, ensures the necessary stiffness of the sealing lip, thereby preventing lateral movement of the sealing lip.
The width profile of the seal can preferably be selected in such a manner thatxe2x80x94if the seal is not yet abradedxe2x80x94the width in the region of the projection increases at increasing distance from the opposite surface to a smaller extent than the width in the region of the main body element increases at increasing distance. As a result, in this case the projection is of more pointed design than the main body element.
Alternatively, the projection can also have an essentially constant width or can even taper at increasing distance. It is particularly advantageous if the width of the projection decreases at increasing distance from the sealing-lip supporting surface. As a result, a loss in surface pressure due to a loss in material because of the decrease in the supporting surface of the sealing lip can is compensated. In addition, the opposite surface is ground smooth at the beginning to a relatively wide track, and so, in consequence, there is less wear of the sealing lip following the abrasion.
In an advantageous refinement, the main body element can have an essentially triangular cross-sectional configuration with two main side surfaces tapering toward the projection. In the case of such a construction of the sealing lip, the projection can simply be integrally formed on the main body element.
According to a further aspect, the sealing lip consists of an essentially elastic or resilient sealing material. Furthermore, the main body element and the projection are preferably designed as a single piece. Since the main body element has substantially more volume than the projection, the elasticity of the main body element essentially comes into play during the contact pressure force, and the contact pressure force decreases only negligibly during wear of the projection. Furthermore, such a construction of a sealing lip can be realized in an extremely simple manner for various sealing arrangements without additional components, coatings, or the like being required.
According to a further advantageous refinement of the invention, at least one of a grinding material is provided in the direction parallel to the sealing-lip supporting surface adjacent to at least the projection. A grinding material of this type assists or accelerates the grinding-in process, with the result that the opposite surface is polished more rapidly, thereby substantially stopping the abrasion of the sealing lip.
Furthermore, according to one embodiment, at least one layer of a supporting material can be provided in the direction parallel to the sealing-lip supporting surface adjacent to at least the projection, said supporting material preferably being softer than the sealing material, at least in the region of the projection. This prevents the sealing lip from moving laterally in the region of the projection. By virtue of the fact that the supporting material is softer, and therefore more elastic than the sealing material of the projection, the surface pressure is concentrated on the sealing-lip supporting region of the projection, with the result that the surface pressure is increased or maintained.
According to a further preferred refinement, the projection has at least one initial sealing lip with the sealing-lip supporting surface bearing against the opposite surface, and at least one additional sealing lip with an additional sealing-lip supporting surface spaced apart from the opposite surface. In this case, the opposite surface is first ground-in by the initial sealing lip. The additional sealing lip then comes into play after appropriate wear of the initial sealing lip. The sealing action of the sealing lip is improved on account of a smaller sealing-lip supporting surface of the additional sealing lip in comparison to the worn away initial sealing lip. To reinforce this effect, a plurality of additional sealing lips and/or a plurality of initial sealing lips may be provided.
In the case of a sealing arrangement having a sealing lip according to the invention, the sealing action can be maintained in spite of wear. In addition, the quality requirements which have to be met by the processing of the opposite surface to reduce the friction of the sealing lip on the opposite surface can be reduced, since, according to the invention, the sealing lip itself grinds in and polishes its running surface on account of a predetermined amount of initial wear. A seal according to the invention is therefore more robust with respect to tolerances during the processing or treatment of the opposite surface. Finally, the extended durability of the seal means that maintenance costs can be reduced.
The invention will be explained in greater detail below by way of example with reference to the figures, in which the same reference numbers are used in the figures for identical or essentially identical elements. In the figures: